The demand for advances in digital camera performance is ever increasing. Consumers wish to capture photos and videos with high resolution and high sensitivity. Improved spatial resolution may be achieved by reducing the area of individual image sensor pixels, such that a greater number of pixels may be accommodated by the same image sensor area. However, a reduction in pixel size generally reduces the light collected by each pixel and thus leads to a decrease in light sensitivity. Decreased light sensitivity adversely affects camera performance in low-light situations, for example nighttime photography, and in capture of dynamic scenes with fast moving objects, such as scenes of sporting events. One solution to this problem, which is implemented in commonly available digital single-lens reflex camera, is to provide an image sensor with a significantly larger area. This allows for incorporating a larger number of pixels without reducing the area of individual pixels. Unfortunately, such sensors are associated with greater cost, both for the image sensor itself and for the imaging objective required for properly imaging a scene onto the enlarged sensor, which precludes use in many applications.
Backside illuminated image sensors offer an alternative solution. In conventional frontside illuminated image sensors light incident on a pixel must pass through a layer of electrical connections before reaching the photosensitive element. This is associated with a loss of light. Backside illuminated image sensors are oriented, seen from the point of view of incident light, such that the layer of electrical connections is located below the photosensitive element. Accordingly, incident light may reach the photosensitive elements without being affected by the electrical connections, which results in greater light collection efficiency and, thus, improved sensitivity.
Color image capture is provided by disposing a color filter on top of the image sensor pixel array. Different pixels are associated with different color coatings, where each type of color coating transmits a certain color. For example, in a Bayer type color image sensor, the color filter includes three different types of color coatings, R, G, and B, configured for transmission of red, green, and blue light, respectively. Color pixels, providing color data, may be formed by grouping together one pixel with R-type coating, two pixels with G-type coating, and one pixel with B-type coating.